We investigated the effects of CsA administration to rats on 1)plasma and urinary levels of ET, 2) expression of ET-1, ET-3 and endothelin converting enzyme (ECE) in renal tissue, 3)clearance of infused 125 I- ET-1, and 4)degradation of 125I-ET-1 by recombinant neutral endopeptidase. Rats given CsA for 6 days developed severe renal insufficiency, e.g., GFR decreased 74%. Urinary excretion of ET increased from an undetectable level to 31.7+6.0pg/24h, while plasma levels of ET were unchanged. Reverse transcription followed by a quantitative polymerase chain reaction (RT-PCR) revealed that ET-1-mRNA in the renal medulla increased 59%, while the expression of both ET-3 and ECE was unchanged. Neither acute nor chronic treatment with CsA affected either the clearance of 125I-ET-1 from the blood or the renal and pulmonary uptake of the peptide. Moreover, CsA did not affect the degradation of 125I-ET-1 by highly purified recombinant neutral endopeptidase, a well known endothelinase. These findings indicate that: 1) the elevated urinary ET levels occurring afer CsA treatment originate from the kidney, and reflect increased renal synthesis of Et, 2) the production of ET is regulated at the level of mRNA transcription, and 3) the expressions of ET-1 and ET-3 are regulated independently.